By Steve Bush 14th April 2022
MIT has created a thermovoltaic demonstrator that can convert heat energy at between 1,900 and 2,400°C in to electricity energy with 40% efficiency – which is above that of a steam turbine. The technology is aimed at future grid-scale energy stores.
Analagous to a multi-junction photovoltaic cell, MIT’s device has two stacked junctions, each picking up part of the incident spectrum.
In this case, the stacked cells are lattice-mismatched AlGaInAs (1.2eV) and GaInAs (1.0eV) – mismatched to the GaAs substrate on which they are grown.
There is a second design, with a matched (and therefore better quality) GaAs (1.4eV) top cell and a mismatched GaInAs (1.2eV) bottom cell.
The 1.2-1.0eV cell has potential for higher power density than the 1.4-1.2eV cell because it converts a broader band of the incident spectrum, while the reduced current density of the 1.4/1.2eV combination will be more efficient (41.1% at 2.39W/cm2, 2,400°C emitter) that the 1.2-1.0 cell (39.3% 1.8Wcm2 2,127°C) when resistive losses are an issue, according to ‘Thermophotovoltaic efficiency of 40%’, a paper in Nature which describes the work.
Which ever junction pait is selected, there is something that is not at all like a solar cell: proximity to the energy source. As the thermoelectric cell will be adjacent to heat source, a mirror (a gold layer in this case) applied to the back of the cell can reflect sub-bandgap (unused) thermal photons back into the heat store (93% of them in this case), reducing the amount of energy that will later be needed to heat it.
The proof-of-concept devices are 10 x 10mm and mounted on a heatsink (see image) to remove unused heat that has not been reflected back.
In a practical grid-scale energy store, around 1,000m2 of thermovoltaic array would convert radiant energy from a super-heated graphite structure, heated by spare renewable energy when it was available.
40% might not seem high as a storage efficiency for those used to the near 100% efficiency of Li-ion cells, but MIT point out that graphite as a storage medium cost little ($0.5/kg). The projected capital cost is <$10/kWh.
“Thermophotovoltaic cells were the last key step toward demonstrating that thermal batteries are a viable concept,” said MIT mechanical engineer Professor Asegun Henry. “This is an absolutely critical step on the path to proliferate renewable energy and get to a fully decarbonized grid.”
He points out that infrastructure created for making large-scale photovoltaic cells could be repurposed to make thermo-voltaic panels.
‘Thermophotovoltaic efficiency of 40%‘ is published in Nature, and can be read in full without payment.
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